Titan is de sixf gravitationawwy rounded moon from Saturn. Freqwentwy described as a pwanet-wike moon, Titan is 50% warger dan Earf's moon and 80% more massive. It is de second-wargest moon in de Sowar System after Jupiter's moon Ganymede, and is warger dan de pwanet Mercury, but onwy 40% as massive. Discovered in 1655 by de Dutch astronomer Christiaan Huygens, Titan was de first known moon of Saturn, and de sixf known pwanetary satewwite (after Earf's moon and de four Gawiwean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii. From Titan's surface, Saturn subtends an arc of 5.09 degrees and wouwd appear 11.4 times warger in de sky dan de Moon from Earf.

Titan is primariwy composed of ice and rocky materiaw, which is wikewy differentiated into a rocky core surrounded by various wayers of ice, incwuding a crust of ice Ih and a subsurface wayer of ammonia-rich wiqwid water.[16] Much as wif Venus before de Space Age, de dense opaqwe atmosphere prevented understanding of Titan's surface untiw de Cassini–Huygens mission in 2004 provided new information, incwuding de discovery of wiqwid hydrocarbon wakes in Titan's powar regions. The geowogicawwy young surface is generawwy smoof, wif few impact craters, awdough mountains and severaw possibwe cryovowcanoes have been found.

The atmosphere of Titan is wargewy nitrogen; minor components wead to de formation of medane and edane cwouds and nitrogen-rich organic smog. The cwimate—incwuding wind and rain—creates surface features simiwar to dose of Earf, such as dunes, rivers, wakes, seas (probabwy of wiqwid medane and edane), and dewtas, and is dominated by seasonaw weader patterns as on Earf. Wif its wiqwids (bof surface and subsurface) and robust nitrogen atmosphere, Titan's medane cycwe is anawogous to Earf's water cycwe, at de much wower temperature of about 94 K (−179.2 °C; −290.5 °F).

Huygens named his discovery Saturni Luna (or Luna Saturni, Latin for "Saturn's moon"), pubwishing in de 1655 tract De Saturni Luna Observatio Nova (A New Observation of Saturn's Moon).[21] After Giovanni Domenico Cassini pubwished his discoveries of four more moons of Saturn between 1673 and 1686, astronomers feww into de habit of referring to dese and Titan as Saturn I drough V (wif Titan den in fourf position). Oder earwy epidets for Titan incwude "Saturn's ordinary satewwite".[22] Titan is officiawwy numbered Saturn VI because after de 1789 discoveries de numbering scheme was frozen to avoid causing any more confusion (Titan having borne de numbers II and IV as weww as VI). Numerous smaww moons have been discovered cwoser to Saturn since den, uh-hah-hah-hah.

The name Titan, and de names of aww seven satewwites of Saturn den known, came from John Herschew (son of Wiwwiam Herschew, discoverer of two oder Saturnian moons, Mimas and Encewadus), in his 1847 pubwication Resuwts of Astronomicaw Observations Made during de Years 1834, 5, 6, 7, 8, at de Cape of Good Hope.[23][24] He suggested de names of de mydowogicaw Titans (Ancient Greek: Τῑτᾶνες), broders and sisters of Cronus, de Greek Saturn, uh-hah-hah-hah. In Greek mydowogy, de Titans were a race of powerfuw deities, descendants of Gaia and Uranus, dat ruwed during de wegendary Gowden Age.

Titan's orbit (highwighted in red) among de oder warge inner moons of Saturn, uh-hah-hah-hah. The moons outside its orbit are (from de outside to de inside) Iapetus and Hyperion; dose inside are Rhea, Dione, Tedys, Encewadus, and Mimas.

Titan orbits Saturn once every 15 days 22 hours. Like Earf's Moon and many of de satewwites of de giant pwanets, its rotationaw period (its day) is identicaw to its orbitaw period; Titan is tidawwy wocked in synchronous rotation wif Saturn, and permanentwy shows one face to de pwanet. Longitudes on Titan are measured westward, starting from de meridian passing drough dis point.[25] Its orbitaw eccentricity is 0.0288, and de orbitaw pwane is incwined 0.348 degrees rewative to de Saturnian eqwator.[6] Viewed from Earf, Titan reaches an anguwar distance of about 20 Saturn radii (just over 1,200,000 kiwometers (750,000 mi)) from Saturn and subtends a disk 0.8 arcseconds in diameter.

The smaww, irreguwarwy shaped satewwite Hyperion is wocked in a 3:4 orbitaw resonance wif Titan, uh-hah-hah-hah. A "swow and smoof" evowution of de resonance—in which Hyperion migrated from a chaotic orbit—is considered unwikewy, based on modews. Hyperion probabwy formed in a stabwe orbitaw iswand, whereas de massive Titan absorbed or ejected bodies dat made cwose approaches.[26]

Titan is 5,149.46 kiwometers (3,199.73 mi) in diameter,[7] 1.06 times dat of de pwanet Mercury, 1.48 dat of de Moon, and 0.40 dat of Earf. Before de arrivaw of Voyager 1 in 1980, Titan was dought to be swightwy warger dan Ganymede (diameter 5,262 kiwometers (3,270 mi)) and dus de wargest moon in de Sowar System; dis was an overestimation caused by Titan's dense, opaqwe atmosphere, wif a haze wayer 100-200 kiwometres above its surface. This increases its apparent diameter.[27] Titan's diameter and mass (and dus its density) are simiwar to dose of de Jovian moons Ganymede and Cawwisto.[28] Based on its buwk density of 1.88 g/cm3, Titan's composition is hawf water ice and hawf rocky materiaw. Though simiwar in composition to Dione and Encewadus, it is denser due to gravitationaw compression. It has a mass 1/4226 dat of Saturn, making it de wargest moon of de gas giants rewative to de mass of its primary. It is second in terms of rewative diameter of moons to a gas giant; Titan being 1/22.609 of Saturn's diameter, Triton is warger in diameter rewative to Neptune at 1/18.092.

Titan is probabwy partiawwy differentiated into distinct wayers wif a 3,400-kiwometer (2,100 mi) rocky center.[29] This rocky center is surrounded by severaw wayers composed of different crystawwine forms of ice.[30] Its interior may stiww be hot enough for a wiqwid wayer consisting of a "magma" composed of water and ammonia between de ice Ih crust and deeper ice wayers made of high-pressure forms of ice. The presence of ammonia awwows water to remain wiqwid even at a temperature as wow as 176 K (−97 °C) (for eutectic mixture wif water).[31] The Cassini probe discovered de evidence for de wayered structure in de form of naturaw extremewy-wow-freqwency radio waves in Titan's atmosphere. Titan's surface is dought to be a poor refwector of extremewy-wow-freqwency radio waves, so dey may instead be refwecting off de wiqwid–ice boundary of a subsurface ocean.[32] Surface features were observed by de Cassini spacecraft to systematicawwy shift by up to 30 kiwometers (19 mi) between October 2005 and May 2007, which suggests dat de crust is decoupwed from de interior, and provides additionaw evidence for an interior wiqwid wayer.[33] Furder supporting evidence for a wiqwid wayer and ice sheww decoupwed from de sowid core comes from de way de gravity fiewd varies as Titan orbits Saturn, uh-hah-hah-hah.[34] Comparison of de gravity fiewd wif de RADAR-based topography observations[35] awso suggests dat de ice sheww may be substantiawwy rigid.[36][37]

The moons of Jupiter and Saturn are dought to have formed drough co-accretion, a simiwar process to dat bewieved to have formed de pwanets in de Sowar System. As de young gas giants formed, dey were surrounded by discs of materiaw dat graduawwy coawesced into moons. Whereas Jupiter possesses four warge satewwites in highwy reguwar, pwanet-wike orbits, Titan overwhewmingwy dominates Saturn's system and possesses a high orbitaw eccentricity not immediatewy expwained by co-accretion awone. A proposed modew for de formation of Titan is dat Saturn's system began wif a group of moons simiwar to Jupiter's Gawiwean satewwites, but dat dey were disrupted by a series of giant impacts, which wouwd go on to form Titan, uh-hah-hah-hah. Saturn's mid-sized moons, such as Iapetus and Rhea, were formed from de debris of dese cowwisions. Such a viowent beginning wouwd awso expwain Titan's orbitaw eccentricity.[38]

A 2014 anawysis of Titan's atmospheric nitrogen suggested dat it has possibwy been sourced from materiaw simiwar to dat found in de Oort cwoud and not from sources present during co-accretion of materiaws around Saturn, uh-hah-hah-hah.[39]

Titan is de onwy known moon wif a significant atmosphere,[40] and its atmosphere is de onwy nitrogen-rich dense atmosphere in de Sowar System aside from Earf's. Observations of it made in 2004 by Cassini suggest dat Titan is a "super rotator", wike Venus, wif an atmosphere dat rotates much faster dan its surface.[41] Observations from de Voyager space probes have shown dat Titan's atmosphere is denser dan Earf's, wif a surface pressure about 1.45 atm. It is awso about 1.19 times as massive as Earf's overaww,[42] or about 7.3 times more massive on a per surface area basis. Opaqwe haze wayers bwock most visibwe wight from de Sun and oder sources and obscure Titan's surface features.[43] Titan's wower gravity means dat its atmosphere is far more extended dan Earf's.[44] The atmosphere of Titan is opaqwe at many wavewengds and as a resuwt, a compwete refwectance spectrum of de surface is impossibwe to acqwire from orbit.[45] It was not untiw de arrivaw of de Cassini–Huygens spacecraft in 2004 dat de first direct images of Titan's surface were obtained.[46]

Energy from de Sun shouwd have converted aww traces of medane in Titan's atmosphere into more compwex hydrocarbons widin 50 miwwion years—a short time compared to de age of de Sowar System. This suggests dat medane must be repwenished by a reservoir on or widin Titan itsewf.[49] The uwtimate origin of de medane in its atmosphere may be its interior, reweased via eruptions from cryovowcanoes.[50][51][52][53][54]

On September 30, 2013, propene was detected in de atmosphere of Titan by NASA's Cassini spacecraft, using its composite infrared spectrometer (CIRS).[57] This is de first time propene has been found on any moon or pwanet oder dan Earf and is de first chemicaw found by de CIRS. The detection of propene fiwws a mysterious gap in observations dat date back to NASA's Voyager 1 spacecraft's first cwose pwanetary fwyby of Titan in 1980, during which it was discovered dat many of de gases dat make up Titan's brown haze were hydrocarbons, deoreticawwy formed via de recombination of radicaws created by de Sun's uwtraviowet photowysis of medane.[47]

On October 24, 2014, medane was found in powar cwouds on Titan, uh-hah-hah-hah.[58][59]

Powar cwouds, made of medane, on Titan (weft) compared wif powar cwouds on Earf (right), which are made of water or water ice.

Titan's surface temperature is about 94 K (−179.2 °C). At dis temperature, water ice has an extremewy wow vapor pressure, so de wittwe water vapor present appears wimited to de stratosphere.[60] Titan receives about 1% as much sunwight as Earf.[61] Before sunwight reaches de surface, about 90% has been absorbed by de dick atmosphere, weaving onwy 0.1% of de amount of wight Earf receives.[62]

Atmospheric medane creates a greenhouse effect on Titan's surface, widout which Titan wouwd be far cowder.[63] Conversewy, haze in Titan's atmosphere contributes to an anti-greenhouse effect by refwecting sunwight back into space, cancewwing a portion of de greenhouse effect and making its surface significantwy cowder dan its upper atmosphere.[64]

Cwouds typicawwy cover 1% of Titan's disk, dough outburst events have been observed in which de cwoud cover rapidwy expands to as much as 8%. One hypodesis asserts dat de soudern cwouds are formed when heightened wevews of sunwight during de soudern summer generate upwift in de atmosphere, resuwting in convection. This expwanation is compwicated by de fact dat cwoud formation has been observed not onwy after de soudern summer sowstice but awso during mid-spring. Increased medane humidity at de souf powe possibwy contributes to de rapid increases in cwoud size.[67] It was summer in Titan's soudern hemisphere untiw 2010, when Saturn's orbit, which governs Titan's motion, moved Titan's nordern hemisphere into de sunwight.[68] When de seasons switch, it is expected dat edane wiww begin to condense over de souf powe.[69]

The surface of Titan has been described as "compwex, fwuid-processed, [and] geowogicawwy young".[70] Titan has been around since de Sowar System's formation, but its surface is much younger, between 100 miwwion and 1 biwwion years owd. Geowogicaw processes may have reshaped Titan's surface.[71] Titan's atmosphere is twice as dick as Earf's, making it difficuwt for astronomicaw instruments to image its surface in de visibwe wight spectrum.[72] The Cassini spacecraft used infrared instruments, radar awtimetry and syndetic aperture radar (SAR) imaging to map portions of Titan during its cwose fwy-bys. The first images reveawed a diverse geowogy, wif bof rough and smoof areas. There are features dat may be vowcanic in origin, disgorging water mixed wif ammonia onto de surface. There is awso evidence dat Titan's ice sheww may be substantiawwy rigid,[36][37] which wouwd suggest wittwe geowogic activity.[73]
There are awso streaky features, some of dem hundreds of kiwometers in wengf, dat appear to be caused by windbwown particwes.[74][75] Examination has awso shown de surface to be rewativewy smoof; de few objects dat seem to be impact craters appeared to have been fiwwed in, perhaps by raining hydrocarbons or vowcanoes. Radar awtimetry suggests height variation is wow, typicawwy no more dan 150 meters. Occasionaw ewevation changes of 500 meters have been discovered and Titan has mountains dat sometimes reach severaw hundred meters to more dan 1 kiwometer in height.[76] This may be compared to de much wider topowogicaw variations found on Earf and Mars, wif Owympus Mons on Mars being 26 km above de surrounding pwains, and Earf's Mauna Kea over 10 km above de ocean fwoor.

Titan's surface is marked by broad regions of bright and dark terrain, uh-hah-hah-hah. These incwude Xanadu, a warge, refwective eqwatoriaw area about de size of Austrawia. It was first identified in infrared images from de Hubbwe Space Tewescope in 1994, and water viewed by de Cassini spacecraft. The convowuted region is fiwwed wif hiwws and cut by vawweys and chasms.[77] It is criss-crossed in pwaces by dark wineaments—sinuous topographicaw features resembwing ridges or crevices. These may represent tectonic activity, which wouwd indicate dat Xanadu is geowogicawwy young. Awternativewy, de wineaments may be wiqwid-formed channews, suggesting owd terrain dat has been cut drough by stream systems.[78] There are dark areas of simiwar size ewsewhere on Titan, observed from de ground and by Cassini; at weast one of dese, Ligeia Mare, Titan's second-wargest sea, is awmost a pure medane sea.[79][80]

Titan mosaic from a Cassini fwyby. The warge dark region is Shangri-La.

Titan in fawse cowor showing surface detaiws and atmosphere. Xanadu is de bright region at de bottom-center.

Titan gwobe, a mosaic of infrared images wif nomencwature.

Titan composite image in infrared. It features de dark, dune-fiwwed regions Fensaw (norf) and Aztwan (souf).

The possibiwity of hydrocarbon seas on Titan was first suggested based on Voyager 1 and 2 data dat showed Titan to have a dick atmosphere of approximatewy de correct temperature and composition to support dem, but direct evidence was not obtained untiw 1995 when data from Hubbwe and oder observations suggested de existence of wiqwid medane on Titan, eider in disconnected pockets or on de scawe of satewwite-wide oceans, simiwar to water on Earf.[81]

The Cassini mission confirmed de former hypodesis. When de probe arrived in de Saturnian system in 2004, it was hoped dat hydrocarbon wakes or oceans wouwd be detected from de sunwight refwected off deir surface, but no specuwar refwections were initiawwy observed.[82] Near Titan's souf powe, an enigmatic dark feature named Ontario Lacus was identified[83] (and water confirmed to be a wake).[84] A possibwe shorewine was awso identified near de powe via radar imagery.[85] Fowwowing a fwyby on Juwy 22, 2006, in which de Cassini spacecraft's radar imaged de nordern watitudes (dat were den in winter), severaw warge, smoof (and dus dark to radar) patches were seen dotting de surface near de powe.[86] Based on de observations, scientists announced "definitive evidence of wakes fiwwed wif medane on Saturn's moon Titan" in January 2007.[87][88] The Cassini–Huygens team concwuded dat de imaged features are awmost certainwy de wong-sought hydrocarbon wakes, de first stabwe bodies of surface wiqwid found outside Earf.[87] Some appear to have channews associated wif wiqwid and wie in topographicaw depressions.[87] The wiqwid erosion features appear to be a very recent occurrence: channews in some regions have created surprisingwy wittwe erosion, suggesting erosion on Titan is extremewy swow, or some oder recent phenomena may have wiped out owder riverbeds and wandforms.[71] Overaww, de Cassini radar observations have shown dat wakes cover onwy a smaww percentage of de surface, making Titan much drier dan Earf.[89] Most of de wakes are concentrated near de powes (where de rewative wack of sunwight prevents evaporation), but severaw wong-standing hydrocarbon wakes in de eqwatoriaw desert regions have awso been discovered, incwuding one near de Huygens wanding site in de Shangri-La region, which is about hawf de size of de Great Sawt Lake in Utah, USA. The eqwatoriaw wakes are probabwy "oases", i.e. de wikewy suppwier is underground aqwifers.[90]

In June 2008, de Visuaw and Infrared Mapping Spectrometer on Cassini confirmed de presence of wiqwid edane beyond doubt in Ontario Lacus.[91] On December 21, 2008, Cassini passed directwy over Ontario Lacus and observed specuwar refwection in radar. The strengf of de refwection saturated de probe's receiver, indicating dat de wake wevew did not vary by more dan 3 mm (impwying eider dat surface winds were minimaw, or de wake's hydrocarbon fwuid is viscous).[92][93]

Near-infrared radiation from de Sun refwecting off Titan's hydrocarbon seas

On Juwy 8, 2009, Cassini's VIMS observed a specuwar refwection indicative of a smoof, mirror-wike surface, off what today is cawwed Jingpo Lacus, a wake in de norf powar region shortwy after de area emerged from 15 years of winter darkness. Specuwar refwections are indicative of a smoof, mirror-wike surface, so de observation corroborated de inference of de presence of a warge wiqwid body drawn from radar imaging.[94][95]

Earwy radar measurements made in Juwy 2009 and January 2010 indicated dat Ontario Lacus was extremewy shawwow, wif an average depf of 0.4–3 m, and a maximum depf of 3 to 7 m (9.8 to 23.0 ft).[96] In contrast, de nordern hemisphere's Ligeia Mare was initiawwy mapped to depds exceeding 8 m, de maximum discernabwe by de radar instrument and de anawysis techniqwes of de time.[96]
Later science anawysis, reweased in 2014, more fuwwy mapped de depds of Titan's dree medane seas and showed depds of more dan 200 meters (660 ft). Ligeia Mare averages from 20 to 40 m (66 to 131 ft) in depf, whiwe oder parts of Ligeia did not register any radar refwection at aww, indicating a depf of more dan 200 m (660 ft). Whiwe onwy de second wargest of Titan's medane seas, Ligeia "contains enough wiqwid medane to fiww dree Lake Michigans".[97]

In May 2013, Cassini's radar awtimeter observed Titan's Vid Fwumina channews, defined as a drainage network connected to Titan's second wargest hydrocarbon sea, Ligeia Mare. Anawysis of de received awtimeter echoes showed dat de channews are wocated in deep (up to ~570 m), steep-sided, canyons and have strong specuwar surface refwections dat indicate dey are currentwy wiqwid fiwwed. Ewevations of de wiqwid in dese channews are at de same wevew as Ligeia Mare to widin a verticaw precision of about 0.7 m, consistent wif de interpretation of drowned river vawweys. Specuwar refwections are awso observed in wower order tributaries ewevated above de wevew of Ligeia Mare, consistent wif drainage feeding into de main channew system. This is wikewy de first direct evidence of de presence of wiqwid channews on Titan and de first observation of hundred-meter deep canyons on Titan, uh-hah-hah-hah. Vid Fwumina canyons are dus drowned by de sea but dere are a few isowated observations to attest to de presence of surface wiqwids standing at higher ewevations.[98]

During six fwybys of Titan from 2006 to 2011, Cassini gadered radiometric tracking and opticaw navigation data from which investigators couwd roughwy infer Titan's changing shape. The density of Titan is consistent wif a body dat is about 60% rock and 40% water. The team's anawyses suggest dat Titan's surface can rise and faww by up to 10 metres during each orbit. That degree of warping suggests dat Titan's interior is rewativewy deformabwe, and dat de most wikewy modew of Titan is one in which an icy sheww dozens of kiwometres dick fwoats atop a gwobaw ocean, uh-hah-hah-hah.[99] The team's findings, togeder wif de resuwts of previous studies, hint dat Titan's ocean may wie no more dan 100 kiwometers (62 mi) bewow its surface.[99][100] On Juwy 2, 2014, NASA reported de ocean inside Titan may be as sawty as de Dead Sea.[101][102] On September 3, 2014, NASA reported studies suggesting medane rainfaww on Titan may interact wif a wayer of icy materiaws underground, cawwed an "awkanofer", to produce edane and propane dat may eventuawwy feed into rivers and wakes.[103]

In 2016, Cassini found de first evidence of fwuid-fiwwed channews on Titan, in a series of deep, steep-sided canyons fwowing into Ligeia Mare. This network of canyons, dubbed Vid Fwumina, range in depf from 240 to 570 m and have sides as steep as 40°. They are bewieved to have formed eider by crustaw upwifting, wike Earf's Grand Canyon, or a wowering of sea wevew, or perhaps a combination of de two. The depf of erosion suggests dat wiqwid fwows in dis part of Titan are wong-term features dat persist for dousands of years.[104]

Photo of infrared specuwar refwection off Jingpo Lacus, a wake in de norf powar region

Radar, SAR and imaging data from Cassini have reveawed few impact craters on Titan's surface.[71] These impacts appear to be rewativewy young, compared to Titan's age.[71] The few impact craters discovered incwude a 440-kiwometer-wide (270 mi) two-ring impact basin named Menrva seen by Cassini's ISS as a bright-dark concentric pattern, uh-hah-hah-hah.[106] A smawwer, 60-kiwometer-wide (37 mi), fwat-fwoored crater named Sinwap[107] and a 30 km (19 mi) crater wif a centraw peak and dark fwoor named Ksa have awso been observed.[108] Radar and Cassini imaging have awso reveawed "crateriforms", circuwar features on de surface of Titan dat may be impact rewated, but wack certain features dat wouwd make identification certain, uh-hah-hah-hah. For exampwe, a 90-kiwometer-wide (56 mi) ring of bright, rough materiaw known as Guabonito has been observed by Cassini.[109] This feature is dought to be an impact crater fiwwed in by dark, windbwown sediment. Severaw oder simiwar features have been observed in de dark Shangri-wa and Aaru regions. Radar observed severaw circuwar features dat may be craters in de bright region Xanadu during Cassini's Apriw 30, 2006 fwyby of Titan, uh-hah-hah-hah.[110]

Many of Titan's craters or probabwe craters dispway evidence of extensive erosion, and aww show some indication of modification, uh-hah-hah-hah.[105] Most warge craters have breached or incompwete rims, despite de fact dat some craters on Titan have rewativewy more massive rims dan dose anywhere ewse in de Sowar System. There is wittwe evidence of formation of pawimpsests drough viscoewastic crustaw rewaxation, unwike on oder warge icy moons.[105] Most craters wack centraw peaks and have smoof fwoors, possibwy due to impact-generation or water eruption of cryovowcanic wava. Infiww from various geowogicaw processes is one reason for Titan's rewative deficiency of craters; atmospheric shiewding awso pways a rowe. It is estimated dat Titan's atmosphere reduces de number of craters on its surface by a factor of two.[112]

The wimited high-resowution radar coverage of Titan obtained drough 2007 (22%) suggested de existence of nonuniformities in its crater distribution, uh-hah-hah-hah. Xanadu has 2–9 times more craters dan ewsewhere. The weading hemisphere has a 30% higher density dan de traiwing hemisphere. There are wower crater densities in areas of eqwatoriaw dunes and in de norf powar region (where hydrocarbon wakes and seas are most common).[105]

Pre-Cassini modews of impact trajectories and angwes suggest dat where de impactor strikes de water ice crust, a smaww amount of ejecta remains as wiqwid water widin de crater. It may persist as wiqwid for centuries or wonger, sufficient for "de syndesis of simpwe precursor mowecuwes to de origin of wife".[113]

Near-infrared image of Tortowa Facuwa, dought to be a possibwe cryovowcano

Scientists have wong specuwated dat conditions on Titan resembwe dose of earwy Earf, dough at a much wower temperature. The detection of argon-40 in de atmosphere in 2004 indicated dat vowcanoes had spawned pwumes of "wava" composed of water and ammonia.[114] Gwobaw maps of de wake distribution on Titan's surface reveawed dat dere is not enough surface medane to account for its continued presence in its atmosphere, and dus dat a significant portion must be added drough vowcanic processes.[115]

Stiww, dere is a paucity of surface features dat can be unambiguouswy interpreted as cryovowcanoes.[116] One of de first of such features reveawed by Cassini radar observations in 2004, cawwed Ganesa Macuwa, resembwes de geographic features cawwed "pancake domes" found on Venus, and was dus initiawwy dought to be cryovowcanic in origin, untiw Kirk et aw. refuted dis hypodesis at de American Geophysicaw Union annuaw meeting in December 2008. The feature was found to be not a dome at aww, but appeared to resuwt from accidentaw combination of wight and dark patches.[117][118] In 2004 Cassini awso detected an unusuawwy bright feature (cawwed Tortowa Facuwa), which was interpreted as a cryovowcanic dome.[119] No simiwar features have been identified as of 2010.[120] In December 2008, astronomers announced de discovery of two transient but unusuawwy wong-wived "bright spots" in Titan's atmosphere, which appear too persistent to be expwained by mere weader patterns, suggesting dey were de resuwt of extended cryovowcanic episodes.[31]

In March 2009, structures resembwing wava fwows were announced in a region of Titan cawwed Hotei Arcus, which appears to fwuctuate in brightness over severaw monds. Though many phenomena were suggested to expwain dis fwuctuation, de wava fwows were found to rise 200 meters (660 ft) above Titan's surface, consistent wif it having been erupted from beneaf de surface.[121]

A mountain range measuring 150 kiwometers (93 mi) wong, 30 kiwometers (19 mi) wide and 1.5 kiwometers (0.93 mi) high was awso discovered by Cassini in 2006. This range wies in de soudern hemisphere and is dought to be composed of icy materiaw and covered in medane snow. The movement of tectonic pwates, perhaps infwuenced by a nearby impact basin, couwd have opened a gap drough which de mountain's materiaw upwewwed.[122] Prior to Cassini, scientists assumed dat most of de topography on Titan wouwd be impact structures, yet dese findings reveaw dat simiwar to Earf, de mountains were formed drough geowogicaw processes.[123] In December 2010, de Cassini mission team announced de most compewwing possibwe cryovowcano yet found. Named Sotra Patera, it is one in a chain of at weast dree mountains, each between 1000 and 1500 m in height, severaw of which are topped by warge craters. The ground around deir bases appears to be overwaid by frozen wava fwows.[124]

Most of Titan's highest peaks occur near its eqwator in so-cawwed "ridge bewts". They are bewieved to be anawogous to Earf's fowd mountains such as de Rockies or de Himawayas, formed by de cowwision and buckwing of tectonic pwates, or to subduction zones wike de Andes, where upwewwing wava (or cryowava) from a mewting descending pwate rises to de surface. One possibwe mechanism for deir formation is tidaw forces from Saturn, uh-hah-hah-hah. Because Titan's icy mantwe is wess viscous dan Earf's magma mantwe, and because its icy bedrock is softer dan Earf's granite bedrock, mountains are unwikewy to reach heights as great as dose on Earf. In 2016, de Cassini team announced what dey bewieve to be de tawwest mountain on Titan, uh-hah-hah-hah. Located in de Midrim Montes range, it is 3,337 m taww.[125]

If vowcanism on Titan reawwy exists, de hypodesis is dat it is driven by energy reweased from de decay of radioactive ewements widin de mantwe, as it is on Earf.[31] Magma on Earf is made of wiqwid rock, which is wess dense dan de sowid rocky crust drough which it erupts. Because ice is wess dense dan water, Titan's watery magma wouwd be denser dan its sowid icy crust. This means dat cryovowcanism on Titan wouwd reqwire a warge amount of additionaw energy to operate, possibwy via tidaw fwexing from nearby Saturn, uh-hah-hah-hah.[31] The wow-pressure ice, overwaying a wiqwid wayer of ammonium suwfate, ascends buoyantwy, and de unstabwe system can produce dramatic pwume events. Titan is resurfaced drough de process by grain-sized ice and ammonium suwfate ash, which hewps produce a wind-shaped wandscape and sand dune features.[126] Titan may have been much more geowogicawwy active in de past; modews of Titan's internaw evowution suggest dat Titan's crust was onwy 10 kiwometers dick untiw about 500 miwwion years ago, awwowing vigorous cryovowcanism wif wow viscosity water magmas to erase aww surface features formed before dat time. Titan's modern geowogy wouwd have formed onwy after de crust dickened to 50 kiwometers and dus impeded constant cryovowcanic resurfacing, wif any cryovowcanism occurring since dat time producing much more viscous water magma wif warger fractions of ammonia and medanow; dis wouwd awso suggest dat Titan's medane is no wonger being activewy added to its atmosphere and couwd be depweted entirewy widin a few tens of miwwions of years. [127]

In 2008 Jeffrey Moore (pwanetary geowogist of Ames Research Center) proposed an awternate view of Titan's geowogy. Noting dat no vowcanic features had been unambiguouswy identified on Titan so far, he asserted dat Titan is a geowogicawwy dead worwd, whose surface is shaped onwy by impact cratering, fwuviaw and eowian erosion, mass wasting and oder exogenic processes. According to dis hypodesis, medane is not emitted by vowcanoes but swowwy diffuses out of Titan's cowd and stiff interior. Ganesa Macuwa may be an eroded impact crater wif a dark dune in de center. The mountainous ridges observed in some regions can be expwained as heaviwy degraded scarps of warge muwti-ring impact structures or as a resuwt of de gwobaw contraction due to de swow coowing of de interior. Even in dis case, Titan may stiww have an internaw ocean made of de eutectic water–ammonia mixture wif a temperature of 176 K (−97 °C), which is wow enough to be expwained by de decay of radioactive ewements in de core. The bright Xanadu terrain may be a degraded heaviwy cratered terrain simiwar to dat observed on de surface of Cawwisto. Indeed, were it not for its wack of an atmosphere, Cawwisto couwd serve as a modew for Titan's geowogy in dis scenario. Jeffrey Moore even cawwed Titan Cawwisto wif weader.[116][128]

Many of de more prominent mountains and hiwws have been given officiaw names by de Internationaw Astronomicaw Union. According to JPL, "By convention, mountains on Titan are named for mountains from Middwe-earf, de fictionaw setting in fantasy novews by J. R. R. Towkien." Cowwes (cowwections of hiwws) are named for characters from de same Towkien works.[129]

Sand dunes in de Namib Desert on Earf (top), compared wif dunes in Bewet on Titan

In de first images of Titan's surface taken by Earf-based tewescopes in de earwy 2000s, warge regions of dark terrain were reveawed straddwing Titan's eqwator.[130] Prior to de arrivaw of Cassini, dese regions were dought to be seas of wiqwid hydrocarbons.[131] Radar images captured by de Cassini spacecraft have instead reveawed some of dese regions to be extensive pwains covered in wongitudinaw dunes, up to 330 ft (100 m) high[132] about a kiwometer wide, and tens to hundreds of kiwometers wong.[133] Dunes of dis type are awways awigned wif average wind direction, uh-hah-hah-hah. In de case of Titan, steady zonaw (eastward) winds combine wif variabwe tidaw winds (approximatewy 0.5 meters per second).[134] The tidaw winds are de resuwt of tidaw forces from Saturn on Titan's atmosphere, which are 400 times stronger dan de tidaw forces of de Moon on Earf and tend to drive wind toward de eqwator. This wind pattern, it was hypodesized, causes granuwar materiaw on de surface to graduawwy buiwd up in wong parawwew dunes awigned west-to-east. The dunes break up around mountains, where de wind direction shifts.

The wongitudinaw (or winear) dunes were initiawwy presumed to be formed by moderatewy variabwe winds dat eider fowwow one mean direction or awternate between two different directions. Subseqwent observations indicate dat de dunes point to de east awdough cwimate simuwations indicate Titan's surface winds bwow toward de west. At wess dan 1 meter per second, dey are not powerfuw enough to wift and transport surface materiaw. Recent computer simuwations indicate dat de dunes may be de resuwt of rare storm winds dat happen onwy every fifteen years when Titan is in eqwinox.[135] These storms produce strong downdrafts, fwowing eastward at up to 10 meters per second when dey reach de surface.

The "sand" on Titan is wikewy not made up of smaww grains of siwicates wike de sand on Earf,[136] but rader might have formed when wiqwid medane rained and eroded de water-ice bedrock, possibwy in de form of fwash fwoods. Awternativewy, de sand couwd awso have come from organic sowids cawwed dowins, produced by photochemicaw reactions in Titan's atmosphere.[132][134][137] Studies of dunes' composition in May 2008 reveawed dat dey possessed wess water dan de rest of Titan, and are dus most wikewy derived from organic soot wike hydrocarbon powymers cwumping togeder after raining onto de surface.[138] Cawcuwations indicate de sand on Titan has a density of one-dird dat of terrestriaw sand.[139] The wow density combined wif de dryness of Titan's atmosphere might cause de grains to cwump togeder because of static ewectricity buiwdup. The "stickiness" might make it difficuwt for de generawwy miwd breeze cwose to Titan's surface to move de dunes awdough more powerfuw winds from seasonaw storms couwd stiww bwow dem eastward.[140]

Around eqwinox, strong downburst winds can wift micron-sized sowid organic particwes up from de dunes to create Titanian dust storms, observed as intense and short-wived brightenings in de infrared.[141]

Titan is never visibwe to de naked eye, but can be observed drough smaww tewescopes or strong binocuwars. Amateur observation is difficuwt because of de proximity of Titan to Saturn's briwwiant gwobe and ring system; an occuwting bar, covering part of de eyepiece and used to bwock de bright pwanet, greatwy improves viewing.[143] Titan has a maximum apparent magnitude of +8.2,[12] and mean opposition magnitude 8.4.[144] This compares to +4.6[144] for de simiwarwy sized Ganymede, in de Jovian system.

The first probe to visit de Saturnian system was Pioneer 11 in 1979, which reveawed dat Titan was probabwy too cowd to support wife.[146] It took images of Titan, incwuding Titan and Saturn togeder in mid to wate 1979.[147] The qwawity was soon surpassed by de two Voyagers.

Titan was examined by bof Voyager 1 and 2 in 1980 and 1981, respectivewy. Voyager 1's trajectory was designed to provide an optimized Titan fwyby, during which de spacecraft was abwe to determine de density, composition, and temperature of de atmosphere, and obtain a precise measurement of Titan's mass.[148] Atmospheric haze prevented direct imaging of de surface, dough in 2004 intensive digitaw processing of images taken drough Voyager 1's orange fiwter did reveaw hints of de wight and dark features now known as Xanadu and Shangri-wa,[149] which had been observed in de infrared by de Hubbwe Space Tewescope. Voyager 2, which wouwd have been diverted to perform de Titan fwyby if Voyager 1 had been unabwe to, did not pass near Titan and continued on to Uranus and Neptune.[148]:94

Even wif de data provided by de Voyagers, Titan remained a body of mystery—a warge satewwite shrouded in an atmosphere dat makes detaiwed observation difficuwt. The mystery dat had surrounded Titan since de 17f-century observations of Christiaan Huygens and Giovanni Cassini was reveawed by a spacecraft named in deir honor.

The Cassini–Huygens spacecraft reached Saturn on Juwy 1, 2004, and began de process of mapping Titan's surface by radar. A joint project of de European Space Agency (ESA) and NASA, Cassini–Huygens proved a very successfuw mission, uh-hah-hah-hah. The Cassini probe fwew by Titan on October 26, 2004, and took de highest-resowution images ever of Titan's surface, at onwy 1,200 kiwometers (750 mi), discerning patches of wight and dark dat wouwd be invisibwe to de human eye.

On Juwy 22, 2006, Cassini made its first targeted, cwose fwy-by at 950 kiwometers (590 mi) from Titan; de cwosest fwyby was at 880 kiwometers (550 mi) on June 21, 2010.[150] Liqwid has been found in abundance on de surface in de norf powar region, in de form of many wakes and seas discovered by Cassini.[86]

Huygens was an atmospheric probe dat touched down on Titan on January 14, 2005,[151] discovering dat many of its surface features seem to have been formed by fwuids at some point in de past.[152] Titan is de most distant body from Earf to have a space probe wand on its surface.[153]

The Huygens probe wanded just off de easternmost tip of a bright region now cawwed Adiri. The probe photographed pawe hiwws wif dark "rivers" running down to a dark pwain, uh-hah-hah-hah. Current understanding is dat de hiwws (awso referred to as highwands) are composed mainwy of water ice. Dark organic compounds, created in de upper atmosphere by de uwtraviowet radiation of de Sun, may rain from Titan's atmosphere. They are washed down de hiwws wif de medane rain and are deposited on de pwains over geowogicaw time scawes.[154]

After wanding, Huygens photographed a dark pwain covered in smaww rocks and pebbwes, which are composed of water ice.[154] The two rocks just bewow de middwe of de image on de right are smawwer dan dey may appear: de weft-hand one is 15 centimeters across, and de one in de center is 4 centimeters across, at a distance of about 85 centimeters from Huygens. There is evidence of erosion at de base of de rocks, indicating possibwe fwuviaw activity. The ground surface is darker dan originawwy expected, consisting of a mixture of water and hydrocarbon ice. The "soiw" visibwe in de images is interpreted to be precipitation from de hydrocarbon haze above.

In March 2007, NASA, ESA, and COSPAR decided to name de Huygens wanding site de Hubert Curien Memoriaw Station in memory of de former president of de ESA.[155]

The bawwoon proposed for de Titan Saturn System Mission (artistic rendition)

There have been severaw conceptuaw missions proposed in recent years for returning a robotic space probe to Titan, uh-hah-hah-hah. Initiaw conceptuaw work has been compweted for such missions by NASA, de ESA and JPL. At present, none of dese proposaws have become funded missions.

The Titan Saturn System Mission (TSSM) was a joint NASA/ESA proposaw for expworation of Saturn's moons.[159] It envisions a hot-air bawwoon fwoating in Titan's atmosphere for six monds. It was competing against de Europa Jupiter System Mission (EJSM) proposaw for funding. In February 2009 it was announced dat ESA/NASA had given de EJSM mission priority ahead of de TSSM.[160]

The proposed Titan Mare Expworer (TiME) was a wow-cost wander dat wouwd spwash down in a wake in Titan's nordern hemisphere and fwoat on de surface of de wake for dree to six monds.[161][162][163] It was sewected for a Phase-A design study in 2011 as a candidate mission for de 12f NASA Discovery Program opportunity,[164] but was not sewected for fwight.[165]

A conceptuaw design for anoder wake wander was proposed in wate 2012 by de Spanish-based private engineering firm SENER and de Centro de Astrobiowogía in Madrid. The concept probe is cawwed Titan Lake In-situ Sampwing Propewwed Expworer (TALISE).[168][169] The major difference compared to de TiME probe wouwd be dat TALISE is envisioned wif its own propuwsion system and wouwd derefore not be wimited to simpwy drifting on de wake when it spwashes down, uh-hah-hah-hah.

Titan is dought to be a prebiotic environment rich in compwex organic compounds,[55][177] but its surface is in a deep freeze at −179 °C (−290.2 °F; 94.1 K) so wife as we know it cannot exist on de moon's frigid surface.[178] However, Titan seems to contain a gwobaw ocean beneaf its ice sheww, and widin dis ocean, conditions are potentiawwy suitabwe for microbiaw wife.[179][180][181]

The Cassini–Huygens mission was not eqwipped to provide evidence for biosignatures or compwex organic compounds; it showed an environment on Titan dat is simiwar, in some ways, to ones hypodesized for de primordiaw Earf.[182] Scientists surmise dat de atmosphere of earwy Earf was simiwar in composition to de current atmosphere on Titan, wif de important exception of a wack of water vapor on Titan, uh-hah-hah-hah.[183][177]

The Miwwer–Urey experiment and severaw fowwowing experiments have shown dat wif an atmosphere simiwar to dat of Titan and de addition of UV radiation, compwex mowecuwes and powymer substances wike dowins can be generated. The reaction starts wif dissociation of nitrogen and medane, forming hydrogen cyanide and acetywene. Furder reactions have been studied extensivewy.[184]

It has been reported dat when energy was appwied to a combination of gases wike dose in Titan's atmosphere, five nucweotide bases, de buiwding bwocks of DNA and RNA, were among de many compounds produced. In addition, amino acids, de buiwding bwocks of protein were found. It was de first time nucweotide bases and amino acids had been found in such an experiment widout wiqwid water being present.[185]

On Juwy 26, 2017, Cassini scientists positivewy identified de presence of carbon chain anions in Titan's upper atmosphere which appeared to be invowved in de production of warge compwex organics.[186]
These highwy reactive mowecuwes were previouswy known to contribute to buiwding compwex organics in de Interstewwar Medium, derefore highwighting a possibwy universaw stepping stone to producing compwex organic materiaw.[187]

In October 2018, researchers reported wow-temperature chemicaw padways from simpwe organic compounds to compwex powycycwic aromatic hydrocarbon (PAH) chemicaws. Such chemicaw padways may hewp expwain de presence of PAHs in de wow-temperature atmosphere of Titan, and may be significant padways, in terms of de PAH worwd hypodesis, in producing precursors to biochemicaws rewated to wife as we know it.[191][192]

Laboratory simuwations have wed to de suggestion dat enough organic materiaw exists on Titan to start a chemicaw evowution anawogous to what is dought to have started wife on Earf. The anawogy assumes de presence of wiqwid water for wonger periods dan is currentwy observabwe; severaw hypodeses postuwate dat wiqwid water from an impact couwd be preserved under a frozen isowation wayer.[193] It has awso been hypodesized dat wiqwid-ammonia oceans couwd exist deep bewow de surface.[179][194] Anoder modew suggests an ammonia–water sowution as much as 200 kiwometers (120 mi) deep beneaf a water-ice crust wif conditions dat, awdough extreme by terrestriaw standards, are such dat wife couwd survive.[180]Heat transfer between de interior and upper wayers wouwd be criticaw in sustaining any subsurface oceanic wife.[179] Detection of microbiaw wife on Titan wouwd depend on its biogenic effects, wif de atmospheric medane and nitrogen examined.[180]

It has been specuwated dat wife couwd exist in de wakes of wiqwid medane on Titan, just as organisms on Earf wive in water.[195] Such organisms wouwd inhawe H2 in pwace of O2, metabowize it wif acetywene instead of gwucose, and exhawe medane instead of carbon dioxide.[181][195] However, such hypodeticaw organisms wouwd be reqwired to metabowize at a deep freeze temperature of −179 °C (−290.2 °F; 94.1 K).[178]

Aww wife forms on Earf (incwuding medanogens) use wiqwid water as a sowvent; it is specuwated dat wife on Titan might instead use a wiqwid hydrocarbon, such as medane or edane,[196] awdough water is a stronger sowvent dan medane.[197] Water is awso more chemicawwy reactive, and can break down warge organic mowecuwes drough hydrowysis.[196] A wife form whose sowvent was a hydrocarbon wouwd not face de risk of its biomowecuwes being destroyed in dis way.[196]

In 2005, astrobiowogistChris McKay argued dat if medanogenic wife did exist on de surface of Titan, it wouwd wikewy have a measurabwe effect on de mixing ratio in de Titan troposphere: wevews of hydrogen and acetywene wouwd be measurabwy wower dan oderwise expected.[195]

In 2010, Darreww Strobew, from Johns Hopkins University, identified a greater abundance of mowecuwar hydrogen in de upper atmospheric wayers of Titan compared to de wower wayers, arguing for a downward fwow at a rate of roughwy 1028 mowecuwes per second and disappearance of hydrogen near Titan's surface; as Strobew noted, his findings were in wine wif de effects McKay had predicted if medanogenic wife-forms were present.[195][197][198] The same year, anoder study showed wow wevews of acetywene on Titan's surface, which were interpreted by McKay as consistent wif de hypodesis of organisms consuming hydrocarbons.[197] Awdough restating de biowogicaw hypodesis, he cautioned dat oder expwanations for de hydrogen and acetywene findings are more wikewy: de possibiwities of yet unidentified physicaw or chemicaw processes (e.g. a surface catawyst accepting hydrocarbons or hydrogen), or fwaws in de current modews of materiaw fwow.[181] Composition data and transport modews need to be substantiated, etc. Even so, despite saying dat a non-biowogicaw catawytic expwanation wouwd be wess startwing dan a biowogicaw one, McKay noted dat de discovery of a catawyst effective at 95 K (−180 °C) wouwd stiww be significant.[181]

As NASA notes in its news articwe on de June 2010 findings: "To date, medane-based wife forms are onwy hypodeticaw. Scientists have not yet detected dis form of wife anywhere."[197] As de NASA statement awso says: "some scientists bewieve dese chemicaw signatures bowster de argument for a primitive, exotic form of wife or precursor to wife on Titan's surface."[197]

In February 2015, a hypodeticaw ceww membrane capabwe of functioning in wiqwid medane at cryogenic temperatures (deep freeze) conditions was modewed. Composed of smaww mowecuwes containing carbon, hydrogen, and nitrogen, it wouwd have de same stabiwity and fwexibiwity as ceww membranes on Earf, which are composed of phosphowipids, compounds of carbon, hydrogen, oxygen, and phosphorus. This hypodeticaw ceww membrane was termed an "azotosome", a combination of "azote", French for nitrogen, and "wiposome".[199][200]

Despite dese biowogicaw possibiwities, dere are formidabwe obstacwes to wife on Titan, and any anawogy to Earf is inexact. At a vast distance from de Sun, Titan is frigid, and its atmosphere wacks CO2. At Titan's surface, water exists onwy in sowid form. Because of dese difficuwties, scientists such as Jonadan Lunine have viewed Titan wess as a wikewy habitat for wife, dan as an experiment for examining hypodeses on de conditions dat prevaiwed prior to de appearance of wife on Earf.[201] Awdough wife itsewf may not exist, de prebiotic conditions on Titan and de associated organic chemistry remain of great interest in understanding de earwy history of de terrestriaw biosphere.[182] Using Titan as a prebiotic experiment invowves not onwy observation drough spacecraft, but waboratory experiments, and chemicaw and photochemicaw modewing on Earf.[184]

It is hypodesized dat warge asteroid and cometary impacts on Earf's surface may have caused fragments of microbe-waden rock to escape Earf's gravity, suggesting de possibiwity of transpermia. Cawcuwations indicate dat dese wouwd encounter many of de bodies in de Sowar System, incwuding Titan, uh-hah-hah-hah.[202][203] On de oder hand, Jonadan Lunine has argued dat any wiving dings in Titan's cryogenic hydrocarbon wakes wouwd need to be so different chemicawwy from Earf wife dat it wouwd not be possibwe for one to be de ancestor of de oder.[204]

Conditions on Titan couwd become far more habitabwe in de far future. Five biwwion years from now, as de Sun becomes a red giant, its surface temperature couwd rise enough for Titan to support wiqwid water on its surface, making it habitabwe.[205] As de Sun's uwtraviowet output decreases, de haze in Titan's upper atmosphere wiww be depweted, wessening de anti-greenhouse effect on de surface and enabwing de greenhouse created by atmospheric medane to pway a far greater rowe. These conditions togeder couwd create a habitabwe environment, and couwd persist for severaw hundred miwwion years. This is proposed to have been sufficient time for simpwe wife to spawn on Earf, dough de presence of ammonia on Titan wouwd cause chemicaw reactions to proceed more swowwy.[206]

^"Titanian" is de written adjectivaw form of bof Titan and Uranus's moon Titania. However, Uranus's moon has a Shakespearean pronunciation wif a short "i" vowew and de "a" of spa: /tɪˈtɑːniən/, whiwe eider spewwing for Titan is pronounced wif dose two vowews wong: /taɪˈteɪniən/.

^Cassini, G. D. (1673). "A Discovery of two New Pwanets about Saturn, made in de Royaw Parisian Observatory by Signor Cassini, Fewwow of bof de Royaw Societys, of Engwand and France; Engwish't out of French". Phiwosophicaw Transactions. 8 (1673): 5178–5185. doi:10.1098/rstw.1673.0003.